The lost wax method is a well known process for producing castings of many types. The lost wax method is commonly used to create castings for dental prosthetics.
As noted in U.S. Pat. No. 4,741,378, issued May 8, 1988, the production of cast dental restorations and prosthetics involves the production of a wax pattern configured to match the desired restoration. This pattern is attached to a support commonly known as a sprue and the sprue is, in turn, secured to a base member.
The sprue, base member and wax pattern are placed within the container which is thereafter filled with investment. After the investment has hardened, it is heated in order to melt the sprue and wax pattern, the melted wax flowing out of the investment through an opening created by the sprue and base, thereby leaving a passage and cavity within the investment.
Molten alloys are then directed through the passageway created by the sprue and into the casting cavity created by the wax pattern. A centrifuge is customarily used to promote flow. As the alloy solidifies, a casting is produced which is essentially a duplicate of the original wax pattern.
As further noted in U.S. Pat. No. 4,741,378, a variety of sprue assemblies have been developed. However, they have a number of drawbacks. One problem encountered relates to the shrinkage of the alloy as it cools. When the molten alloy filling the casting cavity cools, shrinkage occurs. In order to avoid irregularities in the final casting, there needs to be a source of molten alloy which can be drawn into the pattern cavity as the contents cool. To provide this source of molten alloy, sprue pins were formed with a bulbous area at a point toward the tip so as to create a reservoir of molten alloy within the investment. This reservoir is very small and does not provide a large amount of molten alloy. The primary reservoir for molten alloy lies outside the mold.
The sprue pin of U.S. Pat. No. 4,741,378 was devised to improve the performance of prior art sprue pins, more particularly by use of a hollow sprue pin having a bulbous extension about it wherein the end of the sprue pin which attaches to the wax pattern gradually decreases in external circumference toward the tip. It is indicated that such an arrangement prevents formation of blockages between the cavity and the relatively small reservoir provided by the sprue pin.
One of the problems inherent in prior art arrangements is waste of metal alloy used to form the casting. These alloys often incorporate precious metals. Principally, but not exclusively, this problem is a result of the use of externally disposed “buttons” or reservoirs of alloy material during the casting operation. These cool relatively quickly and material can stick to the mold, producing blockages and otherwise interfere with proper casting.
As will be seen below, the system of the present invention utilizes a relatively large internal reservoir within the confines of the investment. This not only reduces waste and saves labor, the arrangement produces a casting which is well defined, accurate and free of internal voids or other defects. Placement of the alloy reservoir within the confines of the investment enables the alloy therein to remain molten for a long period of time, promoting replenishment of the alloy in the dental prosthetic casting to be supplemented during cooling thereof over that long period of time. There is no shrinkage, porosity or “suck back.” This results in a superior casting. The invention may be used to produce dental prosthetic castings of virtually any shape and size.